Presently, many medical procedures with the goal of providing fluids to a patient's body through external tubing make use of detection mechanisms to monitor the presence of undesirable elements such as gaseous bubbles in the provided fluid. For example, one such commonly used procedure is for conducting dialysis. During dialysis, a patient's blood is generally circulated extracorporeally through an artificial kidney machine, such as a dialysis machine, where harmful and other undesirable elements in the blood are largely filtered from the blood. The filtered blood is then returned to the patient's body, generally through tubing connected directly to a blood vessel. The returned blood, however, may still contain undesirable elements, such as undissolved gaseous bubbles or columns of air that can be harmful if allowed to enter a patient's body. In order to prevent or minimize gaseous bubbles from entering the body, a detection device is commonly used to monitor the blood for the gaseous bubbles prior to the bubbles entering the patient's body. An example of one such air-bubble detector is set forth in U.S. Pat. No. 5,583,280, the disclosure of which is herein incorporated by reference.
Currently, ultrasonic air bubble detectors are used for monitoring blood for gaseous bubbles and other undesirable elements. The details of one such ultrasonic air bubble detector are set forth in U.S. Pat. No. 5,394,732 to Johnson et al, the disclosure of which is herein incorporated by reference.
Conventional ultrasonic air bubble detectors generally transmit an ultrasonic wave from a transmitter through the tubing containing the flowing blood. An ultrasonic wave receptor/detector collects the transmitted wave at the opposite side of the tubing and the waveform is then translated into a signal and analyzed. The analysis generally involves a study of the changes in the ultrasonic waveform characteristics, such as attenuation, resulting from passage through a fluid medium, such as blood. These changes are then compared to predetermined settings indicating the presence of gaseous bubbles in the blood. Other changes in the blood affecting propagation of the ultrasonic wave, such as increased or decreased blood density, are also analyzed and fed back to the transmitter. The transmitter then re-calibrates various waveform parameters, such as intensity and/or frequency, to account for any changes in the blood, thus enabling the detector to continuously detect gaseous bubbles.
In addition to fluid changes, other factors may also affect and/or compromise bubble detection capabilities using ultrasound. For example, it is generally well known that sound waves are susceptible to noise, both ambient and internal. As a result, there exists the potential that any noise detected by the receiver, together with the waveform signal, may cause an erroneous bubble-detection reading. In addition, a sound wave's relatively large wavelength may limit a detector's degree of precision in detecting and/or measuring bubble sizes. In particular, small bubbles of air, for example on the order of several micro-liters, may flow through the tubing undetected by the detector and enter the patient's circulatory system. Such an occurrence would obviously be very harmful, and likely fatal, to the patient.
Although presently available bubble detection devices are well accepted by the medical profession, it is desirable to have a detector that can further minimize and better detect the number of bubbles that may be entering the body of a patient. In particular, it is desirable to have a detector that can detect smaller bubbles of air and with greater degree of precision, while providing for faster recalibration of the detector in the event of sudden changes in the fluid medium.